1. Field of the Invention
The present invention relates to a production method of an oxide superconductive film with a smooth surface, and a high homogeneity in terms of the thickness. Furthermore, the present invention relates to a production method of an oxide superconductive film having excellent crystal properties and epitaxial properties, and good microwave characteristics.
2. Description of the Related Art
Nowadays, studies have been conducted on the electric power application and the electron device application, such as a superconductive lead, a superconductive wire, and a superconductive microwave element, using an oxide high temperature superconductor. As conditions in such an application, (1) a high zero resistance temperature, (2) a high critical current value, and (3) a large area, a homogeneity, and a surface smoothness are required. In order to meet the (2) condition, a thick film needs to be formed, and a high critical current density value is desired with the film thickness.
In order to meet the demands, a 1 .mu.m thickness film formed by an eutectic vacuum evaporation method and a sputtering method is applied with a high temperature heat treatment in an oxygen atmosphere in a conventional method. However, the critical current density at 77K thereof is 10.sup.5 A/cm.sup.2, which is about 1/50 in comparison with the maximum value of a thinner film.
Recently, Foltyn et al reported that a 6 .mu.m thickness Y.sub.1 Ba.sub.2 Cu.sub.3 Ox (YBCO) thin film formed on a CeO.sub.2 /YSZ (100) substrate by a pulse laser deposition method has a critical current density of 1.times.10.sup.6 A/cm.sup.2 in Applied Physics Letters Vol. 63, No. 13, Par. 1848-1850. It is reported that the substrate has a complicated multi-layer structure, and that the film formation rate of the pulse laser deposition method is as slow as; 0.1 to 0.2.mu.m/min.
On the other hand, Kitamura et al reported that a 10 .mu.m thickness Y.sub.1 Ba.sub.2 Cu.sub.3 Ox film formed on 1 MgO (100) substrate by a liquid phase epitaxial growth method treated with a heat treatment at 450.degree. C. for 5 hours in an oxygen atmosphere after the film formation has a critical current density of 1.times.10.sup.5 A/cm.sup.2 at 77K in Physical C Vol. 256, Par. 64-72. Since this film formation method has a high growth rate of about 2 .mu.m/min and a high critical current value proportional to the product of the film thickness and the critical density value, it is promising as a film formation technique. However, since a Ba--Cu oxide as a part of a molten liquid adheres on the film, it is disadvantageous in terms of the surface smoothness and the homogeneity for a superconductive film (superconductive wire, superconductive microwave line) as an electron device.
Furthermore, nowadays studies have been conducted on the application of an electron device using an oxide high temperature superconductor. Above all, research and development are actively conducted on superconductive microwave passive elements (resonator, filter) according to the rapid development proceeding in the mobile communication field.
As conventional methods for producing superconductive films comprising the devices, a pulse laser deposition method, a sputtering method, a eutectic vacuum evaporation method, and a metalorganic chemical vapor deposition method are known. These films are reported to have a comparative good surface resistance value in a microwave region, for example, in Journal of Superconductivity Vol. 6, No. 3, Par. 119-160. However, these films also have problems to be solved. In general, it is known that a large intermodulation distortion in a high frequency device such as a resonator and a filter causes interference in the communication system. Therefore, in a resonator or a filter using a superconductive film, the intermodulation distortion needs to be minimized.
It is reported that the intermodulation distortion of an oxide superconductive film is reported to be strongly dependent on the production method and the production conditions, for example, in IEEE Transactions on Applied Superconductivity Vol. 7, No. 2, Par. 1911-1916. Although the existence of the grain boundary in a film is pointed out as the source thereof, it is reported that the alleviation thereof is difficult.
On the other hand, since a liquid phase epitaxial growth method of a Y.sub.1 Ba.sub.2 Cu.sub.3 Ox film have growth conditions including a low supersaturation and a substantial thermal equilibrium, a high quality single crystalline film is reported to be produced at a high rate in, for example, Journal of Crystal Growth Vol. 158, Par. 61-67. Further, a liquid phase growth Y.sub.1 Ba.sub.2 Cu.sub.3 Ox film is reported to have a very small intermodulation distortion of the surface resistance in the above-mentioned microwave region in, for example, Extended Abstracts of 6th International Superconductive Conference No. 2, J16.
However, the surface resistance value thereof is about 7 to 8 times as high as the value of films formed by a sputtering method and a pulse laser deposition method.